There are many applications for powdered polymeric particles where it is important that the particles have a narrow size distribution. One such application is where the particles are used as electrostatographic toners. In such toners the particles can function, for example, as the sole toner component to form toner images or as binders for other toner addenda such as colorants and charge control agents. The electrostatographic toners are in the form of particles that are subject to electrostatic forces and other forces that affect the particles differently depending on their size, and, in order to obtain good copies, it is necessary that all of the particles be affected in substantially the same way in the copying process. This means that the particles must have a narrow size distribution. While there are many processes that produce polymer particles, few produce such particles having a narrow size distribution. If the particles do not have a narrow size distribution, it may be necessary to size them by passing the particles through sieves. This is an expensive process which significantly adds to the cost of the electrostatographic toner.
A well-known process that provides polymer particles having a narrow size distribution employs a solid colloidal stabilizer to control both particle size and particle size distribution. One example of this type of process is described in U.S. Pat. Nos. 2,932,629 and 4,148,741, which pertain to a suspension polymerization process where a solid colloidal stabilizer such as silica is used to limit the coalescence of droplets containing polymerizable monomer in an aqueous medium. In that process, a water-immiscible polymerizable liquid is sheared to form small droplets suspended in aqueous medium containing a water dispersible water-insoluble solid colloid such as silica as the suspension stabilizer. The concentration and size of the colloid determines the size of the droplets. The colloid performs this function by adhering to the droplets at the water/monomer interface to form a layer on the surface of the droplets. After monomer droplets have coalesced with other droplets and have grown to a particular diameter, the presence of the layer of colloidal stabilizer particles on the surface of the droplets prevents them from further coalescing and increasing in diameter. In this way, all of the droplets tend to grow to approximately the same diameter, so that upon polymerization the resulting polymer particles have a narrow size distribution.
A second example of a process that provides polymer particles having a narrow size distribution using a solid colloidal stabilizer comprises forming a solution of polymer in a solvent that is immiscible with water, dispersing the polymer/solvent solution in an aqueous medium containing silica as the solid colloidal stabilizer, removing the solvent, dehydrating and drying the resulting particles. For ease in distinguishing this type of process from the aforementioned "suspension polymerization" process, it is referred to hereinafter as the "polymer suspension" process. This type of process is described in copending U.S. application Ser. No. 171,065, filed Mar. 21, 1988 in the name of Nair, Pierce and Sreekumar, titled "Polymeric Powders Having A Predetermined and Controlled Size and Size Distribution", and assigned to the same assignee as this application, (now U.S. Pat. No. 4,833,060, issued May 23, 1989) and herein incorporated by reference.
The use of solid colloidal stabilizers such as silica to control particle size and size distribution of the resulting polymers has some disadvantages. For example, such solid colloidal particles can impart surface characteristics to the polymers that are incompatible with the intended use. Thus, if silica is used as the colloidal stabilizer in the preparation of polymer particles for use as electrostatographic toners it must be removed from the particles because silica adversely affects the triboelectric properties and the fixing characteristics of the toner. The removal of silica from the polymer particles requires several additional processing steps that significantly add to the cost of the toner. Furthermore, stabilizers such as silica have a constant composition and, therefore, the surface characteristics of polymer particles coated with such stabilizers cannot be changed. It would be advantageous to use a solid colloidal stabilizer whose composition can be varied so that the surface characteristics of polymer particles prepared using the stabilizer could be tailored to meet specific requirements. This would be particularly advantageous in the preparation of polymer particles for use in electrostatographic toners where it is often necessary to tailor the surface characteristics of the toner to achieve optimal performance upon fixing or transfer of the toner particles. Moreover, solid colloidal stabilizers such as silica require the use of promoters to drive them to the interface between the droplets and the aqueous medium. The use of a solid colloidal stabilizer that would not require such a promoter would greatly simplify the process in which the stabilizer is used.
It is, therefore, evident that there is a need for a suspension process that uses a solid colloidal stabilizer in preparing polymeric particles which stabilizer is not subject to the disadvantages described previously herein. It is also evident that there is a need for polymeric particles that can be prepared in suspension processes and have surface characteristics that are tailored to specific end uses, e.g., as electrostatographic toners. This invention meets these needs.